In hydrodynamic and hydraulic sciences, free surface flows are important but they are difficult to simulate. Smoothed Particle Hydrodynamics (SPH) is a meshfree, Lagrangian particle method for modeling a free surface flow phenomenon such as dam break. One of the problems in SPH method is the time consuming calculations. It needs computers with strong CPU and memory. In this paper, the influence of the time scale in a free surface flow simulation by SPH method is investigated. At first, dam break in different geometrical scales are simulated by SPH method. One of them is considered as hypothetical prototype and the others are reduced scale models. Then the influence of time scale is evaluated in hypothetical prototype and reduced scale models. The results show that the occurrence time of dam break in reduced scale model is less than other models and it?s affected to decrease calculation time in SPH. Therefore, the computer calculation time is decreased. The relations between two scales in dam break simulations indicate that the Froude law is valid for this case. In models with scale 3 and 6 with equal number of particles, the times of occurrence and computation were decreased in the order of 42 and 59 , respectively. Hence, the computer run time can be decreased when this new scheme is used. For validating the SPH model, the numerical results are compared with other numerical methods and experimental data. ? 2015, Korean Society of Civil Engineers and Springer-Verlag Berlin Heidelberg.